Tachyarrhythmias are abnormal heart rhythms characterized by a rapid rate, typically expressed in units of beats per minute (bpm), that can originate in either the ventricles or the atria. Examples of tachyarrhythmias include atrial tachyarrhythmias such as atrial flutter and atrial fibrillation (AF), and ventricular tachyarrhythmias such as ventricular tachycardia (VT), and ventricular fibrillation (VT). The most dangerous tachyarrhythmias are those that have their origin in the ventricles, namely ventricular tachycardia and ventricular fibrillation. Ventricular rhythms occur when re-entry of a depolarizing wavefront in areas of the ventricular myocardium with different conduction characteristics becomes self-sustaining or when an excitatory focus in the ventricle usurps control of the heart rate from the normal physiological pacemaker of the heart, the sino-atrial node. The result is rapid contraction of the ventricles out of electromechanical synchrony with the atria. Most ventricular rhythms exhibit an abnormal QRS complex in an electrocardiogram (ECG) because they do not use the specialized conduction system of the ventricles, the depolarization spreading instead from the excitatory focus or point of re-entry directly into the myocardium. In ventricular tachycardia, the ventricles contract rapidly and produce distorted QRS complexes in an ECG. Ventricular fibrillation, on the other hand, occurs when the ventricles depolarize at an even more rapid rate and in a chaotic fashion, resulting in QRS complexes of constantly changing shape and virtually no effective pumping action.
Implantable cardiac rhythm management devices may be configured to treat both atrial and ventricular tachyarrhythmias with electrical therapy. Devices known as implantable cardioverter/defibrillators (ICDs) deliver an electric shock to the heart which terminates the tachyarrhythmia by depolarizing all of the myocardium simultaneously and rendering it refractory. The most dangerous tachyarrhythmias are ventricular tachycardia and ventricular fibrillation, and ICDs have most commonly been applied in the treatment of those conditions. Both ventricular tachycardia and ventricular fibrillation are hemodynamically compromising, and both can be life-threatening. Ventricular fibrillation, however, causes circulatory arrest within seconds, is the most common cause of sudden cardiac death, and is usually treated with immediate delivery of a defibrillation shock. Ventricular tachycardia can be treated with either a defibrillation or a cardioversion shock, the latter referring to a shock delivered synchronously with an R wave. Another type of electrical therapy for ventricular tachycardia is antitachycardia pacing (ATP). In ATP, the ventricles are competitively paced with one or more pacing pulses in an effort to interrupt the reentrant circuit causing the tachycardia. ATP therapy can successfully treat VT, but it is not effective in terminating VF. Modern ICDs incorporate ATP capability so that ATP therapy can be delivered to the heart when a ventricular tachycardia is detected. Although cardioversion/defibrillation will also terminate ventricular tachycardia, it consumes a large amount of stored power from the battery and results in patient discomfort owing to the high voltage of the shock pulses. It is desirable, therefore, for the ICD to use ATP to terminate a tachyarrhythmia whenever possible. In most ICDs with ATP capability, VF is distinguished from VT using a rate-based criterion so that ATP or shock therapy can be delivered as appropriate, where the heart rate is determined by measurement of the time interval between successive ventricular depolarizations. In a typical device, a tachyarrhythmia with a heart rate in the VT zone is treated with ATP therapy in order to avoid an unnecessary painful shock to the patient, and a defibrillation shock is delivered if the heart rate is in the VF zone or if ATP pacing fails to terminate a tachyarrhythmia in the VT zone.
ICDs are also capable of detecting atrial tachyarrhythmias, such as atrial fibrillation and atrial flutter, and delivering a cardioversion shock pulse to the atria in order to terminate the arrhythmia. Although not immediately life-threatening, it is important to treat atrial fibrillation for several reasons. First, atrial fibrillation is associated with a loss of atrio-ventricular synchrony which can be hemodynamically compromising and cause such symptoms as dyspnea, fatigue, vertigo, and angina. Atrial fibrillation can also predispose to strokes resulting from emboli forming in the left atrium. Although drug therapy and/or in-hospital cardioversion are acceptable treatment modalities for atrial fibrillation, ICDs configured to treat atrial fibrillation offer a number of advantages to certain patients, including convenience and greater efficacy.
As noted above, ICDs detect tachyarrhythmias by measuring the time intervals between successive depolarizations of the atria or ventricles. Situations arise, however, where such devices are subjected to externally produced oscillating electromagnetic fields, referred to as electromagnetic interference or EMI, which are sensed by sensing electrodes and falsely interpreted as cardiac depolarizations. If the frequency at which the externally produced field oscillates is within a range similar to that of a tachyarrhythmia, inadvertent triggering of anti-tachyarrhythmia therapy, such as anti-tachycardia pacing or delivery of a cardioversion/defibrillation shock, can occur. One example of such a situation is during a surgical operation where the electro-cauterizing instruments used to control bleeding can produce EMI that triggers the delivery of anti-tachyarrhythmia therapy by the device. It is therefore common practice to de-activate such anti-tachyarrhythmia functions in ICDs when the patient is expected to be exposed to such electromagnetic interference. De-activating a device before a surgical operation, imaging procedure, or other event and then re-activating it afterwards, however, requires the use of an external programmer in both instances and can be inconvenient. It may even pose a risk to the patient if the re-activation is not done promptly. The present invention is directed toward an improved method and device for dealing with this problem.